Accordingly, the invention is applicable to the growth of SiC, group III-nitrides and all types of alloys thereof. The common problem of growing such objects of a high crystalline quality and at a reasonable grow rate from the commercial point of view will now by way of a non-limitative example, be further explained with respect to SiC.
SiC single crystals are, in particular, grown for use in different types of semiconductor devices such as, for example, different types of diodes, transistors and thyristors intended for applications in which it is possible to benefit from the superior properties of SiC in comparison with especially Si, namely the capability of SiC to function well under extreme conditions. The large band gap between the valence band and the conduction band of SiC makes devices fabricated from this material capable to operate at high temperatures, namely up to 1000 K.
However, high temperatures are needed to obtain a well ordered growth thereof. The epitaxial growth of silicon carbide by Chemical
Vapor Disposition is therefor carried out in a temperature regime of 1400-1700.degree. C. These high temperatures are needed both to obtain decomposition by cracking of the Si- and C-containing precursor gases of the gas mixture and to ensure that the atoms are deposited on the substrate surface in an ordered manner. However, high temperatures also give rise to problems with impurities coming out of different types of material, so that the temperature could until now, not be raised above such temperature interval, thereby resulting in such a low grow rate (some .mu.m per hour) that it is out of the question to grow boules for forming i.a. substrates by using CVD. Accordingly, this method is used only for growing objects in the form of layers.
However, it is not possible to even grow layers of SiC by CVD using already known devices at such a high grow rate that a commercial production thereof is of considerable interest. A raise of temperature has not been tried, since that would have resulted in rapid degradation of the walls of the susceptor due to increased etching of hot spots thereof, leading to unacceptable incorporation of impurities therefrom into the layers grown. It is also assumed that crystalline imperfections reduce the minority carrier lifetime. A high crystalline perfection may be obtained at high temperatures or at low grow rates. The minority carrier lifetime governs the forward conduction losses and switching losses of a power device. The minority carrier lifetime is also reduced by the introduction of unwanted compensating acceptors in the N-type layer grown, so that a minority carrier lifetime acceptable for high-power bipolar devices could not be obtained until now. It is therefore important to produce layers of a high crystalline perfection at high growth rates without the introduction of compensating acceptors. Common impurities as nitrogen and oxygen may also be incorporated at lower and at higher growth rates which also may positively influence the minority carrier lifetime.
As already mentioned, it is due to low growth rates that is impossible to grow boules, which require grow rates in the order of millimeters per hour, by CVD, so that the seeded sublimation technique is presently used for growing boules, which may then be sliced to substrates. However, the crystalline quality of the boules grown by this technique is low in comparison with that of the SiC layers epitaxially grown by CVD. The substrates produced in this way are perforated by small holes called micropipes or pinholes, which limit the device area considerably. For this reason, high-power devices of SiC are not yet of commercial interest.
In the seeded sublimation technique, the source is a SiC powder that sublimes, whereupon the gas species are transported by a temperature gradient to the seed crystal where the growth occurs. The conditions in the gas phase are governed by thermodynamics only, which makes it difficult to keep the C/Si ratio constant due to Si vapor leakage out of the system. Furthermore, the purity of the gases required for Chemical Vapor Deposition are several orders of magnitude higher than that of the source material used for seeded sublimation growth.